opsin

is shown in red. Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become retinylidene proteins, but are usually still called opsins regardless. Most prominently, they are found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction cascade. Another opsin found in the mammalian retina, melanopsin, is involved in circadian rhythms and pupillary reflex but not in vision. Humans have in total nine opsins. Beside vision and light perception, opsins may also sense temperature, sound, or chemicals.

is shown in red. Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become retinylidene proteins, but are usually still called opsins regardless. Most prominently, they are found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction cascade. Another opsin found in the mammalian retina, melanopsin, is involved in circadian rhythms and pupillary reflex but not in vision. Humans have in total nine opsins. Beside vision and light perception, opsins may also sense temperature, sound, or chemicals. The first Pineal Opsin (Pinopsin) was found in the chicken pineal gland. It is a blue sensitive opsin (λ max = 470&nbsp;nm). Pineal opsins have a wide range of expression in the brain, most notably in the pineal region. The first parapinopsin (PP) was found in the parapineal organ of the catfish. The parapinopsin of lamprey is a bistable UV-sensitive opsin (λ max = 370&nbsp;nm) that stays active until it is illuminated by green (500 nm) light. The switch-like behavior makes it attractive as an optogenetic tool. The teleosts have two groups of parapinopsins, one is sensitive to UV (λ max = 360-370&nbsp;nm), the other is sensitive to blue (λ max = 460-480&nbsp;nm) light. The first parietopsin was found in the photoreceptor cells of the lizard parietal eye. The lizard parietopsin is green-sensitive (λ max = 522&nbsp;nm), and despite it is a c-opsin, like the vertebrate visual opsins, it does not induce hyperpolarization via a Gt-protein, but induces depolarization via a Go-protein. The first invertebrate panopsin was found in the ciliary photoreceptor cells of the annelid Platynereis dumerilii and is called c(iliary)-opsin. This c-opsin is UV-sensitive (λ max = 383&nbsp;nm) and can be tuned by 125&nbsp;nm at a single amino-acid (range λ max = 377–502&nbsp;nm). Thus, not unsurprisingly, a second but cyan sensitive c-opsin (λ max = 490&nbsp;nm) exists in Platynereis dumerilii. The first c-opsin mediates in the larva UV induced gravitaxis. The gravitaxis forms with phototaxis a ratio-chromatic depth-gauge. In different depths, the light in water is composed of different wavelengths: First the red (> 600&nbsp;nm) and the UV and violet (< 420&nbsp;nm) wavelengths disappear. The higher the depth the narrower the spectrum so that only cyan light (480&nbsp;nm) is left. Thus, the larvae can determine their depth by color. The color unlike brightness stays almost constant independent of time of day or the weather, for instance if it is cloudy. Panopsins are also expressed in the brains of some insects. The panopsins of mosquito and pufferfish absorb maximally at 500&nbsp;nm and 460&nbsp;nm, respectively. Both activate in vitro Gi and Go proteins. The panopsins are sister to the TMT-opsins. Microbial opsins are often known by the rhodopsin form of the molecule, i.e., rhodopsin (in the broad sense) = opsin + chromophore. Among the many kinds of microbial opsins are the proton pumps bacteriorhodopsin (BR) and xanthorhodopsin (xR), the chloride pump halorhodopsin (HR), the photosensors sensory rhodopsin I (SRI) and sensory rhodopsin II (SRII), as well as proteorhodopsin (PR), Neurospora opsin I (NOPI), Chlamydomonas sensory rhodopsins A (CSRA), Chlamydomonas sensory rhodopsins B (CSRB), channelrhodopsin (ChR), and archaerhodopsin (Arch). Several microbal opsins, such as proteo- and bacteriorhodopsin, are used by various bacterial groups to harvest energy from light to carry out metabolic processes using a non-chlorophyll-based pathway. Beside that, halorhodopsins of Halobacteria and channelrhodopsins of some algae, e.g. Volvox, serve them as light-gated ion channels, amongst others also for phototactic purposes. Sensory rhodopsins exist in Halobacteria that induce a phototactic response by interacting with transducer membrane-embedded proteins that have no relation to G proteins. Microbal opsins (like channelrhodopsin, halorhodopsin, and archaerhodopsin) are used in optogenetics to switch on or off neuronal and cardiac activity . Microbal opsins are preferred if the neuronal activity should be modulated at higher frequency, because they respond faster than animal opsins. This is because microbal opsins are ion channels or proton/ion pumps and thus are activated by light directly, while animal opsins activate G-proteins, which then activate effector enzymes that produce metabolites to open ion channels.

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